The present invention relates to a lithographic technique applied for manufacturing semiconductor devices and more specifically to an exposure monitor mask which monitors with high speed and high accuracy an exposure setting value to achieve a maximum exposure margin and a method of exposure adjustment.
In the process of lithography in the manufacture of semiconductor devices or integrated circuits, apparatuses for making pattern exposure, called exposure equipment, have been used. There is a type of exposure equipment which is called as a reduced projection exposure equipment (stepper, or step and scan exposure system). With this stepper, light from a light source passes through a mask on which an exposure pattern is formed, then is reduced in size by an optical system and projected onto a semiconductor wafer.
In transferring circuit patterns on a mask onto the wafer, it has been required that the patterns capable of being transferred be made as fine as possible. From the optical image formation theory, as is well known, the resolution (linewidth) R and the depth of focus DOF are given by the following equations: EQU R=k1.lambda./NA (1) EQU DOF=k2.lambda./NA.sup.2 (2)
where NA is the numerical aperture of the projection optical system, .lambda.is the wavelength of the projected light, and k1 and k2 are process coefficients.
The above equations are referred to as Rayleigh's equations and have been employed as a measure of evaluation of the image formation performance of projection exposure equipment. To meet the fine patterning requirements, a shorter wavelength has been used for exposure and the numeral aperture of the projection lens has been increased and, at the same time, improvements have been made in process. However, the requirements for scaling down the dimensions of devices have become increasingly rigorous in recent years and it has become difficult to obtain sufficient margins of exposure and depth of focus, which has resulted in a reduction in yield. For this reason, in order to make effective use of a small exposure margin and prevent a reduction in yield, more accurate control for exposure and depth of focus has been demanded.
The control of exposure is usually performed based on measurements of the linewidth in a pattern. The pattern linewidth varies not only with exposure but also with the depth of focus. The finer the pattern, the less negligible the effect of focus errors on the pattern linewidth becomes. It is therefore difficult to determine whether variations in pattern linewidth are due to variations in exposure or variations in depth of focus, causing difficulties in controlling exposure with accuracy.
To solve these problems, there has been made a proposal for an exposure monitor mask having a pattern in which focus errors have no effect on the linewidth (see SPIE Vol. 1261 Integrated Circuit Metrology, Inspection, and Process Control IV (1990) p. 315). The feature of this proposal lies in forming on a wafer a monitor pattern having an exposure profile which does not depend on the focus state by using a mask pattern having a pitch that cannot be resolved by projection exposure equipment and progressively varying the dimensional ratio (the duty ratio) between adjacent light transmitting and blocking portions in the pattern.
In FIG. 19, there is illustrated an example of a monitor pattern based on linewidth measurement. In this figure, P denotes the pattern pitch having a measure under the critical resolution, 1401 denotes light blocking portions, and 1402 denotes light transmitting portions. The monitor pattern is arranged such that the width of the light transmitting portions increases in each direction away from the pattern center with the pattern pitch fixed.
In FIG. 20, there is illustrated the image intensity distribution on that line of the wafer which corresponds to the A-A' portion of the monitor pattern shown in FIG. 19 as the result of transfer of the monitor pattern. Since only zero-order diffracted light diffracted by the monitor pattern is directed onto the wafer, the image intensity varies in proportion to the square of the area of the light transmissive portion of the monitor pattern of FIG. 19. It is expected that, by transferring the mask pattern onto a resist while changing exposure in steps, measuring resulting linewidths on the resist by a linewidth measurement instrument, and calibrating the exposures and the linewidths, it will be possible to monitor exposure with high accuracy using the linewidth measuring instrument.
However, although the linewidth values obtained by measurements are reliable in terms of relative comparison, they have little reliability as absolute values. Therefore, the determination of exposure using the linewidth values themselves as a criterion of the exposure leads to a problem of reliability of exposure.